Recent work on the development of monolithic semiconductor ring lasers is presented. Circular or oval resonant waveguide rings have been demonstrated; the minimum waveguide bend radius dictates the type of guiding structure needed. In very small rings the laser material must be etched through to provide sufficient refractive index contrast and optical guiding; in larger structures weaker guiding using strip loading is adequate, and problems of wall damage and optical scattering can be avoided. However, there may still be residual loss because of the penetration of the guided optical wave above the thinned cladding layer. A fabrication method has been developed to prevent the interaction of the wave with lossy contact metallization. MMI couplers are ideal output devices for larger ring lasers, efficient operation of which has been demonstrated. These laser configurations are also extremely suitable for mode-locked operation; the ring cavity length determines the pulse frequency. Mode-locking at 28 GHz was inferred from the lasing spectrum of a 3.1 mm circumference ring laser, the design of which was modified by the isolation of short lengths of waveguide to form saturable absorbers.

Deep surface grating structures make possible the fabrication of DFB and DBR structures where the usual epitaxial regrowth processes which compromise device yield and reliability are avoided. A key requirement is that the gratings are etched to a well-controlled depth position close to the waveguide core. This paper describes the fabrication processes for the grating/stripe waveguide structures in both DFB lasers with gratings exterior to a central stripe (effectively providing refractive index confinement) and DBR lasers with gratings etched into the central ridge region. Issues of etch depth precision, grating pattern definition using either electron beam lithography or holography and measurement of the grating coupling coefficient, K, are addressed. Both pulsed and CW measurements of DFB laser performance have been carried out including lasers with a novel (lambda) /4 shift. In the DBR lasers, quantum well intermixing via impurity free vacancy disordering has been used to reduce the optical absorption in the unpumped region below the reflector grating. A direction extension of this intermixing approach will allow the development of a more general waveguide-based integration technology in which DFB and DBR lasers are combined with passive waveguide sections and other discrete devices to form a complete photonic chip. The prospects for successful implementation of this integration discussed and an example given using a surface grating DFB laser.

A Cellular Automata (CA) approach is proposed for a microscopic description of elementary matter-radiation interaction processes, which provides an equal opportunity treatment both of matter and radiation state. In particular the case of a semiconductor laser device is investigated. The description of this simple laser structure is preliminary to extending the CA approach to more complex matter-radiation interactions as those deriving from electrostriction and Kerr effects.

Modulation of carrier multiplication factor in a reverse bias p—n junction can be realized through optical generation of carrier on either side of depletion layer which causes injection of photon—current along with thermal reverse saturation current into the denletion layer. The mm—wave characteristics of GaAs homo junction and GaAs/GaJnAs heterojunction in IMPATT and MITATT modes have been comDuted through device simulation process for wide range of values of carrier multiplication factor. The results indicate that the heterojunction provides considerably high efficiency compared to that of homo structure. The aprreciahle change i.n the ralue of diode negative resistance with photon injection (for low value of multiplication factor) would result in a fall in rf rower delivery from the diode. This property of changes in the out put power with change in multiplication factor can be used for detection of optical signal falling in the GaAs layer of both the homo and heterostructure diodes.
Keywords : IMPATT ,MITATT,Heterostructure,Homostructure,Negative resistance.

The bandgap of GaInAsP multi-quantum well (MQW) material can be accurately tuned by photo-absorption induced disordering (PAID) to allow lasers, modulators and passive waveguides to be fabricated from a standard MQW laser structure. The bandgap tuned lasers are assessed in terms of threshold current density, internal quantum efficiency and internal losses and exhibit blue shifts in the lasing spectra of up to 160 nm. The ON/OFF ratios of the modulators were tested over a range of wavelengths with modulation depths of 20 dB obtained from material which has been bandgap shifted by 120 nm, while samples shifted by 80 nm gave modulation depths as high as 27 dB. We have also measured single mode waveguide losses over a range of wavelengths and these are 5 dB/cm at 1550 nm. These high quality devices showing good electrical and optical properties after processing demonstrate that PAID is a promising technique for the integration of devices to produce photonic integrated circuits.

A new differential topology of optical receiver which is based on a differential sensing of the output current of the photodiode is presented in this paper. The receiver proposed here is made by a P-I-N photodiode and two amplifiers, each one amplifies the current furnished by a lead of the photodiode. It is intended for digital transmission systems which require a low-pass type response. By adopting a wide band transimpedance amplifier, this topology can increase the sensitivity of the optical receiver by 3 dB as well as the output level which is increased by 6 dB, and therefore reduces the gain requirement to the transimpedance amplifier and to the main amplifier.

In this work are presented the design parameters and performance of a guided-wave spectrum analyzer based on a multilayered ZnO/AlxGa1-xAs/AlyGa1-yAs/GaAs structure for heterodyne detection of spread spectrum signals. The adopted circuit configuration includes a double integrated collimating grating having non linear groove profiles, an acousto-optic Bragg cell, a concave frating lens, a focusing grating lens and an output photodiode array. The optical structure has been optimized as a function of the layer thickness, and Al title in order to obtain improved performance of the circuit in terms of 3-dB Bragg bandwidth (up to 185 MHz), surface acoustic wave power consumption (less than 3 mW), frequency resolution (less than 1.5 MHz), and single-tone dynamic range (about 56 dB). The number of channels has been found equal to 112 and 125 in the two structures under investigation, respectively. The calculated single- and double-tone dynamic range are reduced of about 40% with respect to the corresponding dynamic ranges of the homodyne architecture. All the design parameters have been calculated for each integrated component of the circuit at the free-space optical wavelength of 0.85 micrometers .

We present the theoretical analysis of a compound configuration of two triple-coupler ring- based resonators (CTCRR) with different radii. Transmittance characteristics of the CTCRR are derived, and their dependence on the amplitude coupling ratios of the directional couplers are studied to determine optimum design relationships. The CTCRR performance is compared to that of a double ring resonator (DRR) configuration reported in literature. It is shown that the CTCRR features double free spectral range (FSR), higher transmission at resonance, and better crosstalk than a DRR with same radii of curvature, waveguide propagation loss, and finesse. The CTCRR results particularly attractive for frequency selective filtering applications in optical FDM systems, where wide FSR systems, where wide FSR and low insertion loss requirements have to be met.

A channel waveguide on an erbium doped phosphate laser glass (Kigre Q89) was fabricated by a dry silver-film ion exchange technique, and its gain properties were studied experimentally. The propagation loss of the fabricated waveguide was 0.6 dB/cm at 1.3 micrometers . Er3+ concentration of 1 X 1020 ions/cm3 was chosen so that no concentration quenching occurred. This was confirmed by measuring a fluorescence lifetime of 1.54 micrometers (8.4 ms). Gain of the fabricated waveguide was measured by using a Ti:sapphire laser at a wavelength of 977 nm and a laser diode of 1.530 micrometers producing pump and signal beams respectively. The signal wavelength used for the experiments was shorter than the emission peak, and the measured gain of the 1.8 cm waveguide was comparable to the total loss. However, the model which adopted experimental conditions showed that lasing is expected at the emission peak with a 3.6 cm long waveguide fabricated by current method. Calculations results showed that the 6.5 dB gain can be realized at 300 mW pump power with a 5.4 cm long waveguide, provided that mode overlap can be increased by 25% and propagation loss can be reduced by 50%. No significant upconversion effect was observed up to 1.1 X 106 W/cm2 pump intensity.

In present work integrated optical refractive index sensor for liquids has been developed. The foundation of measurement method is the change of coupling efficiency of waveguide grating which is coated with unknown refractive index liquid.

We describe the fabrication and properties of optical waveguides formed in Z-cut proton exchanged LiTaO3 and X- and Z-cuts annealed proton exchanged LiNbO3 structures by reverse proton exchange Li+ yields H+ in the LiNO3 and LiNO3-KNO3-NaNO3 melts at 280 - 340 degree(s)C. Surface waveguides, supporting ordinary polarization modes, as well as the buried waveguides for extraordinary polarization modes have been produced. The diffusion constant Do and the activation energy Q for the reverse proton exchange process in LiTaO3 were calculated to be 2.95 (DOT) 107 micrometers 2/h and 97.66 kJ/mol, respectively. A surface decrease of the ordinary refractive index (Delta) noequals -0.034 in H:LiTaO3 waveguides with (Delta) neequals 0.026 was estimated from direct measurements. The reverse proton exchange method was used to form buried waveguides in the well annealed proton exchanged lithium niobate waveguides. The different crystal phases were identified in APE LiNbO3. The relationships between extraordinary and ordinary index changes in APE LiNbO3 waveguides annealed at different temperatures (330 degree(s)C and 400 degree(s)C) were obtained.

We describe a simple low temperature method to produce integrated optics devices in a photosensitive, hybrid, organically modified sol-gel silica glass. In particular, we report on fabrication and characterization of slab and channel waveguides, waveguides with grating and a directional coupler. The fabrication process is appealing for its simplicity, entailing few steps and utilizing elementary photodefinition to give robust, mechanically rigid devices.

A 1:2 Micro-Opto-Mechanical switch has been achieved using the combination of 2 technologies: `Integrated Optics' and `Micromachining on Silicon'. The commutation is obtained by means of the mechanical deflection of a cantilever beam driven by an electrostatic force. The first devices show promising characteristics.

The effect of shunt-shunt feedback in monolithic transimpedance amplifiers for multiGbit per second optical receivers on the input impedance is considered in this paper. The main transfer function to be optimized is the transimpedance, but the input impedance should be taken into careful consideration to avoid unexpected peaking effects which can be a cause of performance degradation. The interconnection technique used to couple the photodiode to the MMIC amplifier has to be properly chosen in conjunction with the performance of the amplifier as current sink at the input port. We propose a design methodology based on the root-locus technique which can help the designer in the optimization of this kind of circuit with particular regard to the input peaking effects.

Fiber/D-fiber splice loss can be minimized by precise axes misalignment. We present an accurate analysis of single-mode fiber/D-fiber splice at the telecommunication wavelengths (lambda) equals 1.3 micrometers and (lambda) equals 1.55 micrometers , by using a combination of vectorial finite element method and mode matching method. Splice loss variation is investigated in terms of fiber axes transverse offset for various D-fiber core-to-flat distances, and the optimum offset that minimizes splice loss is determined. The effect of fiber-axes rotation is also investigated for non null offsets. Fiber axes positioning tolerances are also evaluated for a given percentual loss variation.

Potassium Titanyl Phosphate, KTiOPO4 or KTP, is an excellent nonlinear optical material. Besides its well known bulk crystal nonlinear optic applications like second harmonic generation in Nd:YAG laser, its large electro-optic coefficient, low dielectric constant, and high optical damage threshold make this material attractive for realizing various electro-optic devices such as modulators and switches. This paper will review the recent developments of optical waveguides and devices fabricated in KTP using the ion exchange process.

We report on fabrication and characterization of channel waveguide lasers, operating near 1.06 micrometers and made in a commercial neodymium-doped glass using thermal silver ion exchange. The slope efficiency and threshold are measured as a function of waveguide width. The waveguide structure with a 7.5 micrometers width shows the best laser characteristics in terms of threshold and slope efficiency. The emitted wavelength, while close to the peak emission wavelength of the bulk glass at 1.059 micrometers , varies with the waveguide width. This behavior is interpreted in terms of the birefringence and the dichroism of the waveguides.

Single-mode nickel-diffused ridge waveguides in lithium niobate were fabricated by a combination of nickel diffusion and proton-exchanged wet etching techniques. The nickel diffusion is carried out at 800 degree(s)C for 1.5 hr and the ridge pattern fabrication process starts with a masked proton exchange to induce domain inversion in lithium niobate and then followed by etching away the exchanged areas with a mixture of hydrofluoric acid and nitric acid. An alternative sequence of ridge pattern fabrication and nickel diffusion is also investigated. The SEM graphs show that smooth waveguide surface is obtained. The measured output optical intensity profiles of the ridge waveguide were also shown.

We propose a two step ion exchange process to minimize the losses in silver ion-exchanged waveguides using aluminum as mask material. In a first step the sample with the Al-layer is treated in sodium nitrate in order to oxidize the aluminum. The second step involves an ion exchange in an AgNO3/NaNO3 salt mixture. We applied this method to prepare strip waveguides in a special glass substrate, BGG31, used for telecommunication devices. The low losses of the strip waveguides in BGG31 are important for applications such as integrated optical laser amplifiers that we suggest in this paper.

Proton exchanged planar optical waveguides have been fabricated and characterized in y-cut 4.5 mol% MgO:LiNbO3 crystals using adipic acid as the proton source. These waveguides exhibit a linear-step index profile with a surface index increase (Delta) n of 0.142 measured at the wavelength of 0.633 micrometers . The activation energy Q and the diffusion constant Do were measured to be 73.74 kJ/mol and 9.96 X 107 micrometers 2/h, respectively. The diffusion rate in y-cut substrates is faster than that in z-cut substrates.

The optical properties and both unstrained lattice constants in the high-index zinc-substituted lithium tantalate waveguides are determined depending on exchange and annealing conditions. The LiTaO3-ZnTa2O6 system clearly exhibits a complex structural chemistry. The two distinct phases have been observed. The dependences of the extraordinary and ordinary indices change on the lattice parameters of the unstrained single crystalline Li1-xZnx/2TaO3 solid solutions in both observed phases were obtained.

It has been shown that the lattice-misfit-induced strains in H:LiNbO3 waveguiding layers strongly modify the phase diagram HNbO3-LiNbO3 obtained for powders. Depending on exchange and annealing conditions and crystal cuts, at least the six phases (alpha) , (beta) -, (gamma) -, (delta) -, (zetz) - and (eta) - HxLi1-xNbO3 can exist in single crystal proton-exchanged layers. The high and low temperature modifications were obtained for (alpha) and (beta) - phases. The relationships between crystal lattice parameters of unstrained HxLi1-xNbO3 solid solutions and extraordinary refractive index change have been experimentally determined for the high and low temperature modifications of (alpha) and (beta) phases in proton-exchanged lithium niobate waveguides.

Impurity free vacancy disordering (IFVD) using dielectric caps to induce intermixing in the GaAs/AlGaAs system is described. Silica is used to promote intermixing whilst strontium fluoride is used as a mask against intermixing. Selective bandgap-widening of GaAs/AlGaAs double quantum well laser material has been used to fabricate monolithic extended cavity strip- loaded waveguide lasers. With a differential shift of 21 nm in the wavelength of the photoluminescence peak, overall losses in the extended cavities were less than 6 cm-1 and a red-shift of the lasing spectrum with increasing passive section length is reported. Electroabsorption optical modulators integrated with passive waveguides have been fabricated using an epitaxial structure identical to that of the laser. At a wavelength of 861.6 nm, devices with a 400 micrometers long modulator section showed ON/OFF ratios greater than 35 dB for a reverse bias voltage of 3 V. A variation of the IFVD technique uses partial area coverage by a strontium fluoride mask under a silica cap to determine the amount of quantum well intermixing. The bandgap can then be varied at will across a wafer. Bandgap tuned lasers were fabricated using this technique. Five distinguishable lasing wavelengths were observed from five selected intermixed regions on a single chip. These lasers showed no significant change in transparency current, internal quantum efficiency or internal propagation loss, which indicates that the material quality was not degraded after intermixing.